1. Field of the Invention
The present invention relates to a cathode-ray tube which includes an electron gun comprising a cathode and a plurality of electrodes and, particularly, to a color cathode-ray tube which includes a a plurality of cathodes corresponding to colors.
2. Description of the Related Art
Heretofore, as an electron gun of a color cathode-ray tube, there has been mainly used an electron gun 50 of a multi-beam single-electron gun system in which a plurality of electron beams EB are intersected with each other within the electron gun and are emitted as shown in FIG. 1A, or an electron gun 60, a so-called inline electron gun of a system in which a plurality of electron beams EB are arrayed substantially in parallel within the electron gun and the electron beams are not intersected with each other within the electron gun and are emitted as shown in FIG. 1B.
In the electron guns 50, 60 of any of the systems, three electron beams EB are generated from three cathodes KR, KG, KB corresponding to a display of the three colors red, green and blue. These electron beams pass through respective electrodes of the electron guns 50, 60 and are introduced into the surface of a fluorescent screen of the cathode-ray tube. These electron beams are introduced into adjacent red, green and blue phosphors.
Specifically, as FIG. 2 shows, an electron gun 73 (50, 60) is disposed in a necked-down portion 72c of a cathode-ray tube assembly 72 formed of a glass, for example, of a cathode-ray tube 71. Three electron beams EB are emitted from the electron gun 73. The three electron beams EB travel through a color-selecting mechanism 75, such as a so-called aperture grille, before being incident onto a fluorescent screen 76 formed on the inner surface of a panel portion 72a of the cathode-ray tube assembly 72. These electron beams have to be incident on phosphors emitting red, green and blue light, which are adjacent to each other, although not shown.
It has been customary that the trajectories of the two side electron beams should be changed by suitable means, and the three electron beams go into convergence so that the side beams, i.e., two electron beams, e.g., electron beams usually corresponding to a red and blue display, may intersect with each other on the color-selecting mechanism 75 disposed ahead of the fluorescent screen 76 as-seen from the side of the electron gun 73.
In the electron gun 50 of the system shown in FIG. 1A, a convergence plate 56 is disposed between a focus lens of the electron gun and the color-selecting mechanism so that a stationary electric field acts on the side beams to (change the trajectories of the side beams, and thereby the three electron beams establish a convergence.
In the electron gun 60 of the system shown in FIG. 1B, a convergence of a plurality of electron beams is established by various systems.
In general, there is used a system in which a so-called dog-bone-like, large, overlapping electrostatic lens electric field is formed in the X direction (horizontal direction) on a main lens formed between a third electrode 63 (G3) and a fourth electrode (G4). That is, its lens action is used to change the convergence of each of the electron beams EB and the trajectories of the side beams, thereby establishing the convergence.
As another example, there is known a method of changing the trajectories of the side beams by displacing the axes of the side beams between the opposing electrodes.
In any of the above methods, it has been customary that the convergence is established by the action of the electric field.
As a method of changing the amperage of each electron beam current in response to a video signal, there is generally used a so-called cathode-drive system for changing a cathode-drive voltage. Precisely, the amperage of an electron-beam current should be changed in order to change a liminance. The amperage of an electron-beam current can be changed by changing the cathode voltage. Specifically, when the luminance increases, the cathode potential should be decreased. When the luminance decreases, the cathode potential should be increased.
Incidentally, a constant high-voltage potential is applied to the fluorescent screen or electrodes of other electron guns regardless of the change of the cathode potential.
However, the energy of an electron beam passing through the main lens changes in response to the change of the cathode potential so that the velocity of an electron beam changes.
Specifically, when the luminance increases, the cathode potential decreases and the potential difference increases relatively so that the velocity of the electron beam increases. So, the trajectories of the side beams change.
FIGS. 3A and 3B show the manner in which the trajectories of the side beams change as the velocity of the electron beam increases. FIG. 3A shows the case of the electron gun of the system shown in FIG. 1A. FIG. 3B shows the case of the electron gun of the system shown in FIG. 1B.
When the velocity of the electron beam increases as described above, in the convergence plate 56 shown in FIG. 3A, or in the overlapping electrostatic lens formed between the third electrode 63 (G3) and the fourth electrode 64 (G4) shown in FIG. 3B, the sensitivity with which the electron beam changes its trajectory as the electron beam is deflected is lowered.
As a consequence, the trajectory of the electron beam changes, as shown arrows in the figures, with the result that the side beams which are coincident with each other at the color-selecting mechanism 75 are displaced from each other.
When the amperage of the electron beams"" currents of red, green and blue increase, a so-called repulsion action influences each electron beam more strongly.
A repulsion action which influences the electron beam 57 for diplaying blue emitted from the cathode KB of the electron gun 50 of the system shown in FIG. 1A will be described with reference to FIG. 4.
The electron beam 57 is influenced by a repulsion action 59 acting on the electron beam from other two electron beams 58 while it travels toward the fluorescent screen 76.
This repulsion action 59 is generated by an electric field acting on the electron beams. The repulsion action 59 becomes strong when the amperage of the respective electron beam""s current increases in order to increase the luminance of each color.
While the electron beam 57 is influenced by a repulsion action 59xe2x80x2 acting on the electron beams within the electron gun 50, this repulsion action is generally weaker than the repulsion action 59 which influences the blue electron beam between the electron gun 50 and the color-selecting electrode 75.
The above-described two actions, i.e., a decrease in the sensitivity when the trajectory of the electron beam is changed in response to the velocity and the repulsion action, cause misconvergences in the same direction, respectively.
This influence becomes remarkable for an image with a higher luminance, such as a teletext, compared with an ordinary broadcasting, for example. Specifically, a color shading occurs and characters are displayed doubly, which, therefore, causes picture quality to be deteriorated.
As a method of solving this problem, there have been proposed a method of adding an auxiliary electrode (see Japanese laid-open patent application No. 9-245667) and a method of correcting by a magnetic field (see Japanese laid-open patent application No. 4-61588).
However, these methods need circuits by which the amperage of the electron beams"" currents for displaying red, green and blue are detected and a voltage applied to the auxiliary electrode or current flowing through a coil disposed outside a necked-down portion is adjusted in response to the change of the amperage of each electron beam current.
Because the frequency of a signal applied to a cathode has a tendency to increase as a cathode-ray tube of a television receiver or the like becomes a high-definition cathode-ray tube, it becomes difficult to correct the trajectories of electron beams in accordance with a change in current amperage.
On the other hand, as proposed in Japanese laid-open patent application No. 8-22149, there is a method in which a coining is provided at the side opposing to a cathode of a first electrode, and the coining is made asymmetric with respect to the axis of the beam aperture of the first electrode by offsetting this coining to the inline arrangement direction.
However, it was confirmed that this method cannot substantially achieve correction effects on the electric-current-level of an electron beam necessary for displaying an ordinary television broadcast.
In order to solve the above problems, it is an object of the present invention to provide a cathode-ray tube capable of obtaining an excellent picture quality by improving the amount in which a convergence is changed as a luminance is changed.
A cathode-ray tube according to the present invention includes an electron gun emitting a plurality of electron beams. The electron gun has a plurality of cathodes arrayed in-line and a first electrode opposing the cathodes. Beam apertures for respective cathodes are bored on the first electrode. Among the beam apertures, apertures through which so-called side beams pass are characterized by being bored inclined by a predetermined angle relative to the cathodes.
According to the above-described arrangement of the present invention, an electrostatic lens formed between a cathode generating a side beam and the first electrode is axial asymmetric.
When the amperage of a beam current is low, i.e., the cathode potential is high, this electrostatic lens is axial asymmetric and has a large curvature. On the other hand, when the amperage of a beam current is large, i.e., the cathode potential is low, this electrostatic lens is axial asymmetric but has a reduced curvature. The curvature of the electrostatic lens changes corresponding to the cathode potential. Accordingly, with the changes in the curvature, the trajectories of the electron beams change. The changes of the curvature of the electrostatic lens act such that the displacement of the electron beam trajectory caused by repulsion is canceled. So, it is possible to correct misconvergence caused when the luminance changes, i.e., the amperage of a beam current changes.